1. Field Of The Invention
The present invention relates to a method for treating an aqueous liquid containing dissolved metals chelated by an alkylenepolyamine polyacetic acid. More particularly, the present invention relates to the treatment of certain liquid waste solutions to remove dissolved heavy metals therefrom.
2. Description Of The Prior Art
The alkylenepolyamine polyacetic acid chelants form a known class of compounds having many members. The most common of these are ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DEPA); and of these, (EDTA) is by far the most widely used compound on a commercial scale. Because the materials are relatively insoluble in the acid form, such compounds are normally used as their soluble salts. The alkali metal (e.g. sodium) salts and the ammoniated salts are the best known.
There are many chelant uses for the alkylenepolyamine polyacetic acids, but one such utility is in cleaning iron oxide containing scale from steam generating equipment. In almost any type of metal equipment in which water is evaporated or heat transfer occurs, insoluble salts deposit upon the surface to form a scale. The composition of the scale will vary depending upon the water which has been used, the type of equipment, the operating temperature and the like. The scale may be very dense or coarse, tightly bound to the base metal or not. In most stream generating units, the scale usually consists of oxides of iron, which include magnetite together with red iron oxide. When the steam generating unit is fabricated using parts containing copper or copper alloys, as for example, in the condenser units, the scale will normally also contain copper and/or copper oxide(s).
The scale also may contain small quantities of "heavy" metals such as nickel, zinc, chromium, lead and the like.
Several investigators have addressed the problem of scale removal. Lesinski described in U.S. Pat. No. 3,308,065 a unique way of removing iron oxide containing scale from ferrous metal surfaces and for passivating the clean surface. Lesinski discovered that ammoniated salts of alkylenepolyamine polycarboxylic acids were particularly efficient in scale removal when used at an alkaline pH (generally above about pH of 8). Ammoniated EDTA used at a pH of from about 8 to about 11, preferably about 9, was said to be a preferred solvent.
Harriman et al. in U.S. Pat. No. 3,438,811 discovered that an aqueous solution of a ferric chelate of an alkylenepolyamine polycarboxylic acid at an alkaline pH was unusually effective in removing copper-containing encrustations or elemental copper from ferrous metal surfaces.
Teumac (U.S. Pat. No. 3,413,160) then described a method of passivating ferrous metal surfaces which has been cleaned using an alkylenepolyamine polycarboxylic acid at an alkaline pH to remove iron-oxide containing scale and/or copper encrustations from steam generating equipment. In this process, an oxidizing agent was added at the end of the cleaning process and the oxidation potential of the aqueous solution was carefully monitored to achieve the desired degree of passivation.
The technology described by Lesinski, Harriman, and Teumac represent the state of the art. All use alkylenepolyamine polycarboxylic acids or salts thereof at an alkaline pH and all generate waste solutions which contain dissolved iron as an iron chelate and other metals with the alkylenepolyamine polyacetic acids or salts.
Waste disposal is a significant problem in today's industrial environment. Thus, the problem of how to safely deal with a liquid waste containing dissolved complexes of iron and/or copper and other heavy metals with alkylenepolyamine polyacetic acid chelants is a question that must be faced after each cleaning job. It has been demonstrated that the liquid waste can be safely and efficiently incinerated, but with increasingly stringent controls on incineration, this is becoming less acceptable in many states. Thus, there exists a strong and increasing need for a safe and efficient method for removing heavy metals from such aqueous liquids prior to disposal.
One solution to the problem is described in Kennedy, Jr. (U.S. Pat. No. 4,629,570) which is capable of removing over 99.9 percent of the dissolved iron and copper present in the waste by a sequential treatment. The pH is first adjusted to at least 12.5, sufficient quantities of a soluble calcium salt are added to facilitate growth of insoluble iron hydroxide and an alkali metal N,N-dimethyl or N,N-diethyldithiocarbamate is added to the waste to remove a significant portion of any copper and any other heavy metals present. The remaining solution, however, may be considered a hazardous waste under current EPA regulations.
It would be desirable to provide a method of removing other heavy metals from the waste whereby the material may be rendered non-hazardous in accordance with current EPA regulations.